Methods and Apparatuses for Recovering Data Packet Flow Control Against Radio Base Station Buffer Run Away

ABSTRACT

Recovery control is provided for recovering data packet flow control between a network node and a radio base station over a radio network interface. The base station communicates with at least one user equipment, UE, over a radio interface and measures information relating to a data transmission rate over the radio interface and an amount of a downlink packet queue in the base station associated with the UE communication. In one embodiment, the base station sends at least some of the measurement information to the network node. A process controller in the network node processes the measurement information to determine a desired amount to be stored in the downlink packet queue. The process controller determines a commanded bit rate to transmit data from the network node to the downlink packet queue based on the determined desired amount and transmits data to the radio base station in accordance with the commanded bit rate.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 14/900,848 filed 22 Dec. 2015, which is a U.S. National Phase Application of PCT/IB2013/001381 filed 28 Jun. 2013. The entire contents of each of the aforementioned applications are incorporated herein by reference.

TECHNICAL FIELD

The technology relates to packet data flow control in a radio communications system.

BACKGROUND

Current cellular systems provide browsing the internet and access to services on the internet. The packet data that the cellular system transmits to the end user in these situations must be handled differently than for circuit-switched speech in that the cellular system should maintain a steady flow of packets between the internet source and the end user so that services are not interrupted. Packet data flow control (referred to simply as flow control) in Wideband Code Division Multiple Access (WCDMA) helps achieve this goal. Flow control in WCDMA is distributed between two nodes: the radio network controller (RNC) and the radio base station (RBS). The RBS is responsible for maintaining a steady flow of packets to the User Equipment (UE) over the air interface to provide a satisfactory user experience, e.g., during web browsing. FIG. 1 is a diagram used to explain WCDMA flow control.

The RNC is responsible for ensuring that data is available in the RBS for transmission, and to facilitate this, the RNC sends Radio Link Control (RLC) data via a Transport Network (TN) to the RBS. The RNC thereby affects the buffer level in the RBS by sending RLC PDU's to the RBS via a TN Frame Protocol (FP). The RBS sends capacity allocations (CA) as input to the RNC, and the RNC typically responds by sending an appropriate amount of data with the specified rate to the RBS over the Iub interface.

Because the RLC protocol is terminated in the RNC and in the UE, any RLC data residing in an RBS queue is seen as delayed by the RLC entities involved in the data transmission between the RNC and the UE. As a result, the RNC will, at poll timer expiry, poll data that has already been transmitted from the RNC to the RBS, but which is still residing in the RBS queue for the UE, and consequently, not yet acknowledged by the UE. Thus, if the amount of data in the RBS Priority Queue (PQ) (a term used in WCDMA) is large and the data rate over the air interface (Uu) is low, then the RLC entity in the RNC will poll the RLC entity in the UE as to whether it has received a particular data packet, even though no data has been lost. Consequently, this will lead to unnecessary retransmissions of RLC packet data units (PDU's) with a poll bit set or poll superfields (POLL_SUFI's).

Another problem is posed by RLC retransmissions. RLC retransmissions delayed by data already buffered in the PQ can lead to multiple requests for the same data even though the previously retransmitted data is already in transit, but still buffered in the RBS PQ. In the delayed retransmissions situation, the UE may, due to timer status prohibit expiry or other status reporting trigger, send additional RLC status reports before the initial retransmission even has been transmitted to the UE over the air interface leading to a situation where multiple copies of the same data will be sent both over the TN and the Uu interface. This results in an inefficient use of the TN and air interface resources since the additional copies of retransmitted RLC PDU's do not contribute to the user experienced throughput since they will be discarded as duplicates by the receiving RLC entity in the UE.

In traditional WCDMA downlink flow control, i.e., for High Speed Downlink Packet Access (HSDPA), the RNC arranges received downlink packets in a queue herein referred to as the RNC queue and transmits packages to the RBS according to the maximum bitrates for transmission over the Tub interface.

A more recent flow control approach, such as Active Queue Management (AQM) flow control or Distributed Active Queue Management (D-AQM), avoids buildup in the RNC queue at the RNC by allowing the RNC to forward data to the RBS as soon as possible. This results in queuing in the RBS and not in the RNC, in contrast to traditional WCDMA flow control which attempts to limit the queue length in the RBS to avoid spurious RLC retransmissions. As a result, the RLC Round Trip Time (RTT) can be expected to vary over a greater range for a TN relying on D-AQM flow control as opposed to a TN that uses traditional flow control. One way to handle this would be to increase the RLC timers that are used to monitor the maximum allowed queue delays, but this would result in poor performance due to lower peak throughput and excessive polling delays in certain scenarios, such as short data transmissions when only the last RLC PDU (which carries the poll) is lost.

While D-AQM attempts to adjust the PQ buffer length in the RBS to the Uu rate, typically targeting a certain buffer length or dwell time, the Uu bit rate for services over the radio interface varies significantly over short periods of time, sometimes resulting in large PQ's and low Uu bit rates. In these instances, it takes time for the RBS to shorten the PQ and adjust to the lower Uu bit rate, leading to an increased RLC RTT and subsequent delay of RLC polls and retransmissions. Consequently, D-AQM may have difficulty in maintaining a desired queue (PQ) length which may lead to wasted bandwidth both in the TN and radio network (RAN) due to the unnecessary transmission of polls and multiple copies of RLC retransmissions.

One objective of flow control is to effectively utilize the air interface between the RBS and the UE since radio spectrum is a scarce resource. If the flow of data packets is interrupted or slowed down unnecessarily, then the user or cell throughput suffers. For this reason, it is important to keep the priority queues (PQs) of the RBS non-empty with a suitable margin (a typical but example margin is 125 ms) to avoid poor user throughput.

Another important flow control objective is to avoid the amount of data buffered in the PQ becoming too large and the packet dwell time in the PQ becoming too long. Stated differently, flow control should aim to keep the PQ in the RBS as short as possible while ensuring that enough data is available to fully utilize the air interface capacity. In addition to the problems already noted above, a large RBS PQ leads to large losses of data at High Speed-Downlink Shared Channel (HS-DSCH) cell changes, which also leads to a large number of RLC retransmissions.

Yet another problem is that aggressive TCP applications do not always respond to traditional flow control schemes such as Active Queue Management (AQM) congestion control based on premeditated packet drops. An example of an aggressive TCP application is one which starts a large number of TCP flows in parallel, each carrying only a small amount of data. In such scenarios, traditional AQM schemes may fail to maintain the RBS buffer within targeted values, leading to an excessive PQ buffer build-up. The effect is long download times as illustrated in FIG. 2, which is a graph showing an example of RBS buffer dwell time when AQM flow control is used in a WCDMA system. When multiple data flows are suddenly started, an AQM buffer dwell time “run away” situation occurs, as pointed out in the Figure.

SUMMARY

The technology in this application includes methods and apparatus that provide recovery flow control for recovering data packet flow control between a network node and a radio base station over a radio network interface.

The network node obtains measurement information relating to a data transmission rate over the radio interface from the radio base station to at least one UE and an amount of a downlink packet queue in the base station associated with the UE communication. A process controller in the network node processes the measurement information to determine a desired amount to be stored in the downlink packet queue. The process controller determines a commanded bit rate to transmit data from the network node to the downlink packet queue based on the determined desired amount. The network node transmits data to the radio base station in accordance with the commanded bit rate. The processing, determining, and transmitting steps may be repeated as needed.

In one example embodiment, the obtaining measurement information includes receiving a message from the radio base station with some or all the measurement information.

In another example embodiment, a used data volume is calculated using the commanded bit rate, e.g., until a next measurement information message is received from the radio base station. One or more data packets that exceed the calculated used data volume are discarded from a buffer in the network node used to store data packets for a data flow associated with the UE.

In example embodiments, the process controller performs linear time invariant process control to determine the commanded bit rate. A reference amount of data is determined using a reference dwell time and the measured bit rate over the radio interface. The determined reference amount of data is provided to the process controller which processes the determined reference amount of data along with the measurement information using linear time invariant process control to determine the commanded bit rate. The commanded bit rate is preferably limited to a non-negative value. The process controller preferably takes into account a first delay associated with transmitting data packets from the network node to the radio base station and a second delay associated with signaling the measurement information from the radio base station to the network node. In one example implementation, the process controller includes a feedback lead-lag controller and a feed forward controller.

The recovery control process is distributed. The radio base station measures information relating to a data transmission rate over the radio interface from the radio base station to the at least one UE and an amount of a downlink packet queue in the base station associated with the UE communication. The base station sends the measurement information to the network node and then receives data from the network node at a commanded bit rate based on the measurement information such that a desired amount of data is stored in the downlink packet queue. Like the network node, the base station may repeat the measuring, sending, and receiving steps one or more times as needed.

In one example embodiment, prior to activating the recovery control method, a normal operation flow control process was used by the radio base station to control the transmission of data to the UE. After deactivating the recovery control method, the base station returns to that normal operation flow control process.

In example embodiments, the recovery control method may be activated when a dwell time for data packets stored in the downlink packet queue exceeds a first dwell time threshold and deactivated when the dwell time for data packets stored in the downlink packet queue is less than a second dwell time threshold or an amount of data in the downlink packet queue is less than a data amount threshold.

In an example implementation, the radio base station communicates with the UE using High Speed Downlink Packet Access (HSDPA), and the other flow control process is based on flow control-based Active Queue Management (AQM) where the radio base station determines the data transmission rate over the radio interface from the radio base station to the at least one UE.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram used to explain WCDMA flow control;

FIG. 2 is a graph showing an example of RBS buffer dwell time when AQM flow control is used in a WCDMA system when multiple data flows are suddenly started;

FIG. 3 is a flow chart illustrating example procedures for a radio base station relating to recovery flow control;

FIG. 4 is a flow chart illustrating example procedures for a network node relating to recovery flow control;

FIG. 5 is a function block diagram showing an example communications system in which the recovery flow control technology may be used;

FIG. 6 is a function block diagram showing an example WCDMA HSDPA type communications system in which the recovery flow control technology may be used;

FIG. 7 is a function block diagram showing an example process control for implementing the recovery flow control in accordance with non-limiting example embodiments;

FIG. 8 is a function block diagram showing an example of a feed forward portion of the process control in FIG. 6;

FIG. 9 shows bode plots of an example feedforward portion (open loop) without a controller, i.e., with G₀(s)=1;

FIG. 10 shows bode plots for an example lead lag controller, designed for a worst case delay, a crossover frequency of 0.65 Hz, and a phase margin of 55 degrees;

FIG. 11 shows bode plots of the open loop system in FIG. 9 with the lead lag control in FIG. 10;

FIG. 12 shows bode plots of a feedback (closed loop) system with the lead lag control in FIG. 10;

FIG. 13 shows a Nyquist plot of a system with lead lag control;

FIG. 14 is a function block diagram showing an example of a feed forward back portion of the process control in FIG. 6;

FIG. 15 a frequency response of the feed forward controller in FIG. 14;

FIG. 16 is a block diagram of an example of a linearly combined feedback and feed forward controller;

FIG. 17 graphs controlled PQT for a lower delay case;

FIG. 18 graphs controlled PQL or buffer data volume/amount for a lower delay case;

FIG. 19 graphs controlled PQT for a higher delay case;

FIG. 20 graphs controlled PQL or buffer data volume/amount for a higher delay case.

DETAILED DESCRIPTION

The following sets forth specific details, such as particular embodiments for purposes of explanation and not limitation. But it will be appreciated by one skilled in the art that other embodiments may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and/or discrete logic gates interconnected to perform a specialized function, etc.) and/or using software programs and data in conjunction with one or more digital microprocessors or general-purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, the technology can additionally be considered to be embodied entirely within any form of computer-readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analog) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions.

In terms of computer implementation, a computer is generally understood to comprise one or more processors or one or more controllers, and the terms computer, processor, and controller may be employed interchangeably. When provided by a computer, processor, or controller, the functions may be provided by a single dedicated computer or processor or controller, by a single shared computer or processor or controller, or by a plurality of individual computers or processors or controllers, some of which may be shared or distributed. Moreover, the term “processor” or “controller” also refers to other hardware capable of performing such functions and/or executing software, such as the example hardware recited above.

It should be understood by the skilled in the art that “UE” is a non-limiting term comprising any wireless device or node equipped with a radio interface allowing at least: transmitting signals in UL and receiving and/or measuring signals in DL. A UE herein may comprise a UE (in its general sense) capable of operating or at least performing measurements in one or more frequencies, carrier frequencies, component carriers or frequency bands. It may be a “UE” operating in single- or multi-RAT or multi-standard mode.

A cell is associated with a base station, where a base station comprises in a general sense any node transmitting radio signals in the downlink (DL) and/or receiving radio signals in the uplink (UL). Some example base stations are eNodeB, eNB, Node B, macro/micro/pico radio base station, home eNodeB (also known as femto base station), relay, repeater, sensor, transmitting-only radio nodes or receiving-only radio nodes. A base station may operate or at least perform measurements in one or more frequencies, carrier frequencies or frequency bands and may be capable of carrier aggregation. It may also be a single-radio access technology (RAT), multi-RAT, or multi-standard node, e.g., using the same or different base band modules for different RATs.

The signaling described is either via direct links or logical links (e.g. via higher layer protocols and/or via one or more network nodes). For example, signaling from a coordinating node may pass another network node, e.g., a radio node.

The example embodiments are described in the non-limiting example context of a WCDMA-based cellular radio system. However, the technology is not limited thereto, and may apply to any radio communications system including for example an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) LTE type system.

The policing and recovery flow control technology in this application ensures that radio base station buffer (queue) lengths are maintained at acceptable values. Under baseline conditions, a normal operation flow control procedure operates to control packet flow in the downlink direction towards a UE for one or more data packet flows. Although flow control is applied to each data flow, only the flow control for one data flow is described to simplify the description. A recovery flow control is triggered under predetermined conditions to avoid an excessive buffer build-up in the radio base station buffer and is preferably removed when the buffer fill state (i.e., the amount of data stored in the buffer for the data flow) reaches an acceptable level. The radio base station first measures the priority queue dwell time (PQT) of packets in the PQ. If the PQT is equal to or below a certain threshold, then no action is taken. However, if the PQT is above that threshold, then the following actions are taken. The radio base station measures (1) the amount of data stored in the base station buffer for the data flow, referred to also as Priority Queue Length (PQL), and (2) a transmitted data rate from the base station to the UE associated with that same data flow. The RBS compares the PQL and PQT to a first set of respective predefined threshold values corresponding to a maximum allowed PQL and a maximum allowed PQT. If both the determined PQL and PQT exceed their respective threshold values, then the radio base station conveys measurement information to a core network or a radio network controller (RNC) node via uplink signaling contained in the uplink frame protocol. In the non-limiting WCDMA example, an RNC is used as the example network node. The RNC uses the data rate and the PQL from a measurement message received from the base station to calculate a Priority Queue Time (PQT). The reception of the information triggers the RNC to initiate recovery flow control. A second set of respective predefined threshold values is used to determine when to deactivate recovery flow control and return to the normal flow control, e.g., flow control orchestrated just at the radio base station. Consequently, these first and second sets of threshold values define the ranges within which recovery flow control is active and limits the flow of data from the RNC to the RBS. So, in contrast to known flow control techniques where the RBS alone determines the bit rate requested from the RNC via capacity allocation control frame messages, the recovery flow control technology in this application is selectively applied under certain conditions. The interaction between the RNC and the RBS in the recovery flow control technology distributes the recovery flow control between the RNC and the RBS.

Some initial definitions are set forth to assist in the description but are not intended to be limiting. For example, the term priority queue is used with the understanding that priority queue encompasses other terms for a data store such as queue, buffer, memory, etc. that is used to temporarily store data in the base station prior to transmission over the radio interface.

PQL—packet queue length—the current data volume or amount measured in bits stored in a priority queue (PQ) in the RBS.

Uu bitrate—the current data rate measured in bits/s at which data bits are leaving the PQ for transmission over the air interface from the RBS to the UE.

PQT—packet queue time—the amount of time measured in seconds an incoming data packet will spend in the PQ, given the current PQ data volume and the current radio transmission (Uu) bitrate from the RBS to a UE. PQT is also known as dwell time.

Accordingly, a distributed method and apparatus perform recovery control when the PQT becomes too large, and when the PQT returns to a normal value, flow control is switched back to a normal flow control, e.g., flow control-based AQM in some WCDMA systems.

FIG. 3 is a flow chart illustrating example procedures for a radio base station relating to recovery flow control in accordance with an example embodiment. In step S1, the RBS measures the current PQT (dwell time in the packet queue) in the RBS. If the PQT does not exceed a first dwell time (PQT) threshold 1, then control returns to step S1. If the PQT exceeds the first dwell time (PQT) threshold 1 (step S2), then the recovery control process is activated, and the RBS starts measuring the current PQ data volume (PQL), which is the amount of a downlink packet queue in the base station associated with the UE communication, and the current Uu bitrate from the RBS to the UE, i.e., the current data transmission rate over the radio interface from the radio base station to the UE (step S3). The RBS signals the measured PQL, PQT, and the Uu bitrate to the RNC (step S4). The RBS receives data from the network node at the commanded bitrate received from the network node (described below) in step S5. The RBS then checks to see if the current PQT<a PQT threshold 2 (a second dwell time threshold) or the current PQ data volume (PQL)<a PQL threshold 3 (a data amount threshold) (step S5). If either condition is met, then the RBS returns to normal flow control and monitors PQT at step S1. If neither condition is met, then the RBS continues with recovery flow control and returns to step S3.

FIG. 4 is a flowchart that illustrates example network node procedures followed by the RNC as the network node in this example. The RNC obtains certain measurement information (step S10) relating to an amount of a downlink packet queue in the base station associated with the UE communication, i.e., PQ data volume or PQL, and a data transmission rate over the radio interface from the radio base station to the UE, i.e., Uu bitrate. A dwell time, i.e., PQT, may be determined using PQT=PQL/Uu bitrate. In one example embodiment, measurement information, e.g., PQL and Uu bitrate, may be measured by and received from the radio base station. In another example embodiment, the RNC may use data transmission rate information already available at the network node, and/or may use a current radio link control (RLC) window size to estimate the PQ data volume instead of receiving either or both rate and volume information from the base station. When the RNC receives a measurement information message from the base station, receipt of that message may activate the recovery flow control process in the RNC. However, other triggers to activate the recovery process may be used. The RNC processes the measurement information to determine a desired amount of data to be stored in the PQ (step S11). The RNC determines a commanded bit rate based on the determined desired buffer amount that is to be used for transmission of data from the RNC to the RBS over the Iub interface (step S12).

In an example embodiment detailed below, that commanded bit rate determination is made using a process control-based controller algorithm, sometimes denoted to as a servo controller algorithm. In example embodiments, the servo controller algorithm is a lead lag controller designed to guarantee stability for a predetermined range of signaling and control delays in the control loop between the RNC and the RBS. The RNC calculates a used data volume using the commanded bitrate unit a next message with measurement information is received from the base station (step S13). The RNC may then discard from a buffer in the RNC for the data flow the data packets exceeding the determined buffer amount (step S14). For example, packets could be discarded from one end such as the top of that RNC buffer. Next, the RNC schedules transmission of SDUs from the RNC queue to the RBS at the commanded bit rate (step S15). A decision is made in step S16 whether a new measurement report is received from the RBS. If so, then the RNC returns to step S11 to compute a new commanded bit rate based on the new measurement information. If not, then the RNC (and RBS) returns to normal flow control operation, e.g., AQM flow control (step S17).

FIG. 5 is a function block diagram showing an example communications system in which the recovery flow control technology may be used. A network node 14, e.g., a core network node or an RNC, is coupled to one or more networks 1 such the internet etc. via a network interface 50 and to one or more radio base stations (RBSs) 18 via an RBS interface 60. The network node 14 includes one or more data processors 52 coupled to the interfaces 50 and 60 as well as to one or more memories 54 that store program instructions 56 and data 58 needed to implement the recovery flow control operations described in this application. The RBS 18 communicates over a network interface 2 with the network node 14 via a network node interface 62. The RBS 18 also communicates with one or more UEs 10 over a radio interface via a radio interface 72 that includes for example one or more radio transmitters, receivers, antennas, etc. The RBS 18 further includes one or more data processors 64 coupled to the interfaces 62 and 72 as well as to one or more memories 66 that store program instructions 68 and data 70 needed to implement the recovery flow control operations described in this application.

FIG. 6 is a function block diagram showing an example WCDMA HSDPA type communications system in which the recovery flow control technology may be used. The RNC 14 is coupled via a transport network (TN) 16 to a radio base station node 18 to convey data packets from end devices 12 to UEs 10 over a radio interface 25. The transport network 16 is an Tub transport network in this example and may include one or more transport nodes (not illustrated) which are interfaced to allow conveying data packets between the RNC 14 and RBS 18. The end devices 12 may correspond to various types of devices, including servers, remote terminals, and further UEs. In addition, a further control node is coupled between the RNC 14 and the end devices 12 and corresponds to a Serving General Packet Radio Service Support Node (SGSN) 16.

As illustrated, different protocols or protocol layers may be used to convey data packets between the end devices and the UEs. FIG. 6 shows an end-to-end protocol implemented between the end devices and the UEs, a retransmission protocol implemented between the RNC 14 and the UEs, and a flow control protocol implemented between the RNC 14 and the RBS 18. The end-to-end protocol is a high layer protocol and, in the illustrated example, may be the of the TCP/IP type (TCP: Transport Control Protocol, IP: Internet Protocol). Other protocol types may be used as well, e.g., UDP (User Datagram Protocol) together with the real-time protocol (RTP) and/or its counterpart, the Real-Time Control Protocol (RTCP) to control the data rate. The retransmission protocol is a lower layer protocol, and in the illustrated example, is the RLC protocol, which is a link layer protocol. According to the retransmission protocol, data packets which were not successfully received are retransmitted, which involves some type of feedback mechanism from the retransmission protocol receiver to the retransmission protocol sender, e.g. by sending acknowledgement packets from the receiver to the sender. FIG. 6 also indicates radio access bearers established between the SGSN node 16 and the UEs. A bearer is considered to be a channel having certain guaranteed transport attributes, e.g. with respect to Quality of Service (QoS).

The RBS 18 includes priority queues (PQs) 22, measurement controllers 24 and 26 for measuring a Uu data rate over the radio interface to a UE a current volume of an associated PQ 22. An overflow detector 26 detects whether a PQ has too high of an associated packet dwell time, in which case, measurements of Uu rate 24 and data volume 26 are initiated. A message generator 30 generates a measurement message when predetermined flow recover conditions are met, such as those described in conjunction with FIG. 3. The measurement message includes measurement information associated with a current PQL, PQT, and Uu date rate for a data flow/PQ. A message transmitter 32 transmits the measurement message from the message generator 30 over the TN 34 to the RNC 14.

The RNC 15 includes packet queues 44 for storing data for the UEs received from the SGSN 16. A message receiver 36 receives measurement messages sent via the TN 34 from the message transmitter 32 and triggers the recovery activator 38 to start recovery flow control for this data flow. The recovery flow controller 40 performs recovery flow control for this flow until deactivated by the recovery activator 38 when the message receiver 36 detects that measurements being sent from the base station message transmitter 32 have stopped. The flow controller 40 sends a commanded data rate determined based on the measurement information using a process control algorithm such as the example described below to the packet transmitter 42. The packet transmitter 42 transmits packets to the RBS 18 at the commanded data rate during recovery flow control for the corresponding data flow. When measurements from the message transmitter 32 have stopped, the normal flow controller 23 is performs normal flow controls.

The inventors designed a recovery flow controller in the context of a process or servo control problem where the controlled signal is steered towards a reference signal. They further determined that the recovery flow controller should also handle a number of constraints of the controlled system, i.e., the PQ in the RBS, that are associated with (1) measurements performed in the RBS, (2) delays associated with signaling these measurements from the RBS to the RNC, (3) the effect of a process control-determined bit rate from the RNC to the RBS, and (4) the need for the process control-determined commanded bit rate to be non-negative.

Constraint (1) concerns a risk that the RBS measurements are subject to some amount of jitter (sampling time instance variations). Delay constraints (2) are associated with the signaling and the effect of the process control-determined bit rate. These delays are not known at the time of design of the recovery flow controller, are time varying, are not measurable within the required on-time of the recovery controller, and have a wide range, e.g., from 10 ms to 130 ms. In fact, the effective delay of the sampling period, the measurement delays, and the commanded data rate signal delay add up, which means that the recovery controller needs to handle an unknown delay. Using non-limiting example delays, the summed delay could be from 10 ms+10 ms+20 ms=40 ms, up to 130 ms+130 ms+20 ms=280 ms. The term handle means that i) performance in terms of settling time to the desired reference signal needs to be as specified for the delays in the above range, and ii) that stability of the controller is ensured for all these delays. Constraint (4) means that any negative control signal needs to be replaced by a nonnegative value. This may adversely impact the recovery controller such that some mitigation may be desirable.

The inventors recognized that the delays are not known and cannot be measured with a high bandwidth, and that there may be jitter. Furthermore, discrete time control builds on accurate sampling or possibly on-line measurement interpolation. Since the former cannot be guaranteed and the latter adds complexity, the example embodiments below employ a continuous time design. The delays, jitter, and sampling are combined together in an effective delay, and then a continuous time controller is designed. This continuous time controller is discretized and operated as if the measurements are not affected by the effective delay, as will be described further below.

Since the delays are unknown and cannot be measured, the recovery controller is designed to be robust to delay variations. Because the delays cause a phase decrease, a feedback part of the example recovery controller is designed for a largest delay possible and to ensure stability and performance for lower delays.

As mentioned, the process control objective is to steer the PQT towards a reference PQT value. There is a closed loop feedback control part of the recovery controller that is based on the measured output from the PQ. It will be described below how PQT and PQL are used in this context. The current transmit data rate for the data flow over the radio interface (the Uu bitrate) is also measured. The current transmit data rate over the radio interface has a direct impact on the PQ, however it cannot be manipulated by the recovery controller. In the automatic process control technology field, such a signal is classified as a measurable disturbance, which can enhance performance by application of feed forward or open loop process control.

The controlled system is intentionally designed for linear control. One benefit of this linearity is that the superposition principle allows for separate design of feedback and feed forward parts of the controller. These separate parts are then combined together to form a combined feedback and feed forward servo controller.

FIG. 7 is a function block diagram showing an example process control for implementing the recovery flow control in accordance with non-limiting example embodiments. The process control output is the commanded bitrate of the data transmission for a data flow from the RNC to the base station. The commanded bitrate transforms to a PQ data volume by integration as is shown in the integrator block 90 in FIG. 7. Integration is a linear and time invariant operation, and a PQT is obtained by the equation

$\begin{matrix} {{{{PQT}_{i}(t)} = \frac{q_{i}^{PQ}(t)}{b_{i}^{uu}(t)}},} & (1) \end{matrix}$

where PQT_(i)(t) is the PQT of flow i at time t, q_(i) ^(PQ)(t) is the PQ data volume of flow i at time t, and where b_(i) ^(uu)(t) is the Uu bitrate of flow i at time t. This means the feedback from PQT_(i)(t) would have to be based on a time varying continuous time model, which the inventors decided was less desirable than the linear time invariant model. The inventors overcame this problem by embedding a linear time invariant controller design in a time varying one. This embedding allows the use of feedback control from a measured q_(i) ^(PQ)(t) (PQL), instead of from a measured PQT_(i)(t) (PQT), the use of a time varying computed PQ data volume reference y_(i,ref)(t) instead of a PQT reference ref_(i) ^(PQT):

y _(i,ref)(t)=ref _(i) ^(PQT) ·b _(i) ^(uu)(t).   (2)

The diagram in FIG. 7 describes the process controller and the controlled system for one data packet flow i, but the technology applies to multiple flows. The Figure illustrates that the controlled system and the controller are distributed between the RBS and the RNC and that the Iub interface is used for data transfer and signaling. The PQT reference ref_(i) ^(PQT) is first multiplied at multiplier 80 with the current Uu bitrate measurement b_(i) ^(uu)(t−T_(i) ^(TN,UL)) to produce the reference value for the PQ data volume, y_(i,ref)(t). This operation and the recovery control algorithm implemented in the process controller 82 occur at the RNC. The process controller 82 receives the PQ data volume reference y_(i,ref)(t), the current measured data volume q_(i) ^(PQ)(t−T^(TN,UL)) and the current measured Uu bitrate b_(i) ^(uu)(t−T^(TN,UL)). Based on these inputs and on internally stored states described below, the controller 82 computes a control signal u_(i)(t), which is the commanded bitrate to be used for data transfer from the RNC to the RBS during recovery flow control.

In FIG. 7, the RBS is denoted as NodeB following the 3GPP standard. The commanded bitrate is then used for scheduling of data transfer over Iub. The transmission uses the transport network (TN). Depending on what sort of TN is used and the load on the TN, there is a delay T_(i) ^(TN,DL) before the transmitted data stream arrives in the RNC and starts to add to the PQ. That delay is represented by the corresponding Laplace transform e^(−sT) ^(TN,DL) 84 in FIG. 7. Similarly, the signaling delays associated with the two measurements are represented with the Laplace transform e^(−s,T) ^(i) ^(TN,UL) 94, 96 in the UL direction. Following the e^(−sT) ^(i) ^(TN,DL) delay 84 is a limiter 96 limiting the commanded bitrate to positive values.

The Node B (RBS) implements the PQ as the integrator 90 represented by the Laplace transform 1/s. The difference between the incoming bitrate b_(i) ^(TN)(t−T^(TN,DL))=u_(i)(t) and the radio interface Uu bitrate b_(i) ^(uu)(t), is integrated to produce the PQ data volume q_(i) ^(PQ)(t). The PQT (dwell time) is obtained by division with b_(i) ^(uu)(t) at divider 92.

In example embodiments, a lead-lag controller is used for the feedback part of the servo controller for flow control recovery. FIG. 8 is a function block diagram showing an example of a feedback portion of the process control in FIG. 6. To describe the control system used to design the feedback controller C(s), where s is the complex variable of the Laplace transform, the following equations are useful:

$\begin{matrix} {{y_{i}(s)} = {{\frac{G_{0}(s)}{1 + {G_{o}(s)}}\left( \frac{y_{i,{ref}}^{PQ}(s)}{D(s)} \right)} = {\frac{G_{0}(s)}{1 + {G_{o}(s)}}{{\overset{\_}{y}}_{i,{ref}}^{PQ}(s)}}}} & (3) \\ {{{G_{0}(s)} = {{G_{1}(s)}{G_{2}(s)}{C(s)}{D(s)}}},} & (4) \end{matrix}$

In (3) and (4), G₀(s) denotes the open loop system, y_(i)(s) is the data volume (PQL) and y _(i,ref) ^(PQ)(s) is the desired data volume. The factors of G₀(s) are the transfer delay transfer function G₁(s), the signaling delay transfer function D₂(s), and the PQ transfer function G₂(s). These quantities are explained above and given by equations (5)-(7).

$\begin{matrix} {{G_{1}(s)} = e^{- {sT}_{i}^{{TN},{DL}}}} & (5) \\ {{G_{2}(s)} = \frac{1}{s}} & (6) \\ {{D(s)} = {e^{- {sT}_{i}^{{TN},{UL}}}.}} & (7) \end{matrix}$

The controller transfer function C(s) is designed to be a lead lag controller here. The transfer function of this controller structure is given by:

$\begin{matrix} {{{C(s)} = {{{C_{lag}(s)}{C_{lead}(s)}} = {K\frac{s + a}{s + \frac{a}{M}}N\frac{s + b}{s + {bN}}}}},} & (8) \end{matrix}$

where the controller parameters K, a, M>1, b and N>1 are determined by standard methods to meet the control objectives (settling time and stability).

The standard design procedure may be outlined as follows. First, based on the requirement of meeting the reference value with a certain accuracy in steady state, a desired level of integrating control in the controller is selected by choosing M. The higher the value of M, the more integral action will result. A value close to 10 is used as an example. Second, the parameter a is chosen to be significantly lower than the crossover frequency, e.g., a=0.05ω_(c), to limit phase loss at the crossover frequency. Third, based on the settling time, a preferred crossover frequency is specified which is defined as the frequency where |G₀(jω_(c))|=1. The result of this choice is a closed loop feedback bandwidth close to the selected crossover frequency. This means that the controller parameters are selected so that this constraint is met. Fourth, based on the stability requirement a desired phase margin φ_(margin) is determined, so that the phase at the crossover frequency is arg(G₀=(G₀(jω_(c)))=−180°+φ_(margin). An example value may be

$\phi_{margin} = {\phi_{margin}^{0} + {\frac{180}{\pi}{\tan^{- 1}\left( {\omega_{c}{M/a}} \right)}} - {\frac{180}{\pi}{\tan^{- 1}\left( {\omega_{c}/a} \right)}}}$

to compensate for the phase loss of the lag link. ω_(margin) ⁰ is a nominal phase margin, for example 60°. Fifth, the phase advance to be provided by the lead link is computed so that arg(G₀(jω_(c)))=−180°+φ_(margin). This means that the sum of phases

$\phi_{sum} = {\frac{180}{\pi}\left( {{\arg \left( {G_{1}\left( {j\; \omega_{c}} \right)} \right)} + {\arg \left( {G_{2}\left( {j\; \omega_{c}} \right)} \right)} + {\arg \left( {D\left( {j\; \omega_{c}} \right)} \right)}} \right)}$

is determined, followed by a computation of the required phase advance as φ_(advance)=−180−φ_(sum)+φ_(margin). Then the parameters of the lead link are computed as

$N = {{\frac{1 + {2{\sin\left( {\frac{\pi}{180}\phi_{{advance}\;}} \right)}} + \left( {\sin\left( {\frac{\pi}{180}\phi_{advance}} \right)} \right)^{2}}{\left( {\cos\left( {\frac{\pi}{180}\phi_{advance}} \right)} \right)^{2}}.b} = {\frac{\omega_{c}}{\sqrt{N}}.}}$

The gain K is determined to give |G₀(jω_(c))|=1 to meet the definition of the crossover frequency. This means that

$K = {\frac{\omega_{c}\sqrt{\omega_{c}^{2} + a^{2}}}{\sqrt{N}\sqrt{\omega_{c}^{2} + \left( {a/M} \right)^{2}}}.}$

A non-limiting example design is depicted in FIGS. 9-13. FIG. 9 shows bode plots of an example feedforward portion (open loop) without a controller, i.e., with G₀(s)=1 and without any compensation, i.e., C(s) is set to 1. The amplitude curve is the same for all delays, represented by the magnitude of G₂(s). The phase curves are different, with the largest phase loss occurring for the case with the maximum total loop delay.

FIG. 10 shows bode plots for an example lead lag controller, designed for a worst case delay, a crossover frequency of 0.65 Hz, and a phase margin of 55 degrees. It has a high low frequency gain which is a consequence of the choice of M. The choice corresponds to a leaky integrator in the controller, ensuring that the stationary error becomes sufficiently small. The phase advance reaches a maximum value at the selected cross over frequency of 0.55 Hz.

FIG. 11 shows bode plots of the open loop system in FIG. 9 with the lead lag control in FIG. 10. Note that the phase is well above −180 degrees at the cross over frequency due to the introduced phase advance.

FIG. 12 shows bode plots of a feedback (closed loop) system with the lead lag control in FIG. 10. There are some moderate resonance peaks that correspond to the repeating poles introduced by the loop delay. However, these are not severe, and the controller is expected to behave well. This is confirmed by the Nyquist plot of FIG. 13. Because all curves are to the right of −1, the closed loop system is stable.

The process controller may be implemented in one or more computers. In this case, the continuous time design needs to be transformed to discrete time using a discretization scheme. One example scheme is Tustin's method where a discrete time difference equation is obtained from a continuous time differential equation by the replacement

$\begin{matrix} {s->{\frac{2}{T}{\frac{1 - q^{- 1}}{1 + q^{- 1}}.}}} & (9) \end{matrix}$

Here T denotes the sampling period (typically 40 ms in the present application), and q⁻¹ denotes the backward shift operator (q⁻¹x(t)=x(t−T)).

By insertion of (9) into (8) and using the control error

e _(i)(t)=y _(i,ref) ^(PQ)(t)−y _(i)(t)   (10)

This results in the following difference equation for the controller computation:

u _(i,fb)(t)=r _(fb,1) u _(i,fb)(t−T)+r _(fb,2) u _(i,fb)(t−2T)+s _(fb,0) e(t)+s _(fb,1) e(t−T)+s _(fb,2) e(t−2T)   (11)

The controller parameters are functions of the continuous time parameters and the sampling period via (9) and are not reproduced here.

FIG. 14 is a function block diagram showing an example of a feed forward portion of the process control in FIG. 6. Standard calculations that use the linearity of the block diagram of FIG. 14 show that the output of the feed forward part of the controller can be written as:

y _(i)(s)=G ₂(s)(G ₁(s)F _(f)(s)D(s)−1)b _(i) ^(uu)(s).   (12)

Here y_(i)(s) is again the data volume of the PQ (PQL), and b_(i) ^(uu)(s) is the (measurable but not controllable) bitrate of flow i. Note that the superposition principle allows neglecting the feedback controller when designing the feed forward part. The objective with the feed forward portion is to cancel the effect of the disturbance on the output; hence, it is desirable that (12) be close to zero. Ideally, this is obtained if the feed forward controller transfer function fulfills.

$\begin{matrix} {{F_{f}(s)} = {\frac{1}{{D(s)}{G_{1}(s)}}.}} & (13) \end{matrix}$

Inserting (5) and (7) in (13) results in

F _(f)(s)=e ^(s(T) ^(i) ^(TN,UL) ^(+T) ^(i) ^(TN,DL) ⁾.   (14)

Equation (14) represents an exact shift of the signal ahead in time, i.e., the feed forward controller is non-causal and not feasible. Therefore, an approximation is needed. A first order Taylor series expansion gives

F _(f)(s)≈1+s(T _(i) ^(TN,UL) +T _(i) ^(TN,DL)),   (15)

which replaces the exact shift ahead in time with a prediction ahead in time. Now, as it stands (15) introduces differentiation of the signal (the s) which is undesirable because high frequency disturbances would be amplified. Therefore, a final adjustment is introduced where (15) is low pass filtered as

F _(f)(s)≈(1+s(T _(i) ^(TN,UL) +T _(i) ^(TN ,DL)))L(s),   (16)

where L(s) is a low pass filter. In a preferred embodiment a second order filter is used i.e.

$\begin{matrix} {{L(s)} = {\frac{\omega_{0}^{2}}{s^{2} + {2{\zeta\omega}_{0}} + \omega_{0}^{2}}.}} & (17) \end{matrix}$

In (17), ξ is the damping of the low pass filter, and ω₀ denotes the bandwidth in terms of the angular resonance frequency of the second order filter.

An example of the design appears in FIG. 15 in terms of a Bode diagram. The example feed forward controller is designed for a bandwidth of 4 Hz. As can be seen from the phase curve, the controller introduces a phase advance, just like a lead link. Further, the feed-forward controller is almost 10 times as fast as the feedback controller, and thus, can handle high frequency parts that the feedback controller cannot.

Again, discretization is performed with Tustin's method (9) and the resulting feed forward regulator becomes

u _(i,ff)(t)=r _(ff,1) u _(i,ff)(t−T)+r _(ff,2) u _(i,ff)(t−2T)+s _(ff,0) b _(i) ^(uu)(t)+s _(ff,1) b _(i) ^(uu)(t−T)+s _(ff,2) b _(i) ^(uu)(t−2T)   (18)

The controller parameters are functions of the continuous time parameters and the sampling period via (9) and are not reproduced here.

The feedback controller, the feed forward controller, and the system have so far been assumed linear. As such, the superposition principle applies, and the effect of feedback and feed forward can be added to create the complete servo controller.

FIG. 16 is a block diagram of an example of a linearly combined feedback and feed forward controller. The diagram is the sum of FIG. 7 and FIG. 12. The control signal is the sum of the feedback and feed forward control signals.

One further nonlinear addition is needed. The linear controller may produce negative control commands during the transient phase producing a negative bitrate from the RNC to the Node B, i.e. data transfer in the reverse direction. To avoid this, the following is introduced:

u=max(u _(min) ,u _(i,fb)(t)+u _(i,ff)(t)),   (19)

where u_(min)≥0 is a preconfigured small minimum bitrate.

To study the performance of the controller, recovery control from an initial high PQ data volume and PQT were studied for a non-limiting example. The results of the simulation appear in FIGS. 17-20. The recovery flow controller in all cases steers the PQT to the desired value of 0.125 ms. As can be expected the control is somewhat oscillating in the long delay case.

There are many advantage of the recovery flow control technology described in this application. One example advantage is that the technology ensures that excessive RBS PQ buffers are avoided while still ensuring that sufficient data is available to fully utilize the radio interface bandwidth. Another example advantage is that the recovery control flow mechanism is flexible given that the setting of thresholds and rate at which feedback information is sent may be tailored to suit various feedback delays and buffer target conditions. Further example advantages include: a reduced risk of excessive PQ buffers, this in turn maximizing the utilization of the air interface and maintaining a high throughput; a reduced need for buffer memory in the RBS; and an improved throughput and end-user experience. In terms of flexibility and application, the technology described in this application applies to any other radio system that can use distributed queuing.

Although the description above contains many specifics, they should not be construed as limiting but as merely providing illustrations of some presently preferred embodiments. Embodiments described herein may be considered as independent embodiments or may be considered in any combination with each other to describe non-limiting examples. Although non-limiting, example embodiments of the technology were described in a WCDMA context, the principles of the technology described may also be applied to other radio access technologies. Indeed, the technology fully encompasses other embodiments which may become apparent to those skilled in the art. Reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural and functional equivalents to the elements of the above-described embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed hereby. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the described technology for it to be encompassed hereby. 

What is claimed is:
 1. A method of operation at a radio network controller, the method comprising: sending data over a transport network to a radio base station for transmission over a radio interface to a user equipment, the data belonging to a data flow targeting the user equipment and accumulating in a buffer at the radio base station in dependence on a difference between a first transmit rate used by the radio network controller for sending data belonging to the data flow to the radio base station over the transport network and a second transmit rate used by the radio base station for transmitting data from the buffer over the radio interface to the user equipment; activating and maintaining a recovery flow control mode at the radio network controller for the data flow responsive to receiving measurement signaling from the radio base station for the data flow, the radio base station being configured to begin sending and continue sending the measurement signaling responsive to the accumulated data exceeding a first defined threshold, each instance of the measurement signaling indicating a current buffered data volume and a current buffer dwell time for the accumulated data in the buffer, and a current value of the second transmit rate; and deactivating the recovery flow control mode at the radio network controller for the data flow responsive to cessation of the measurement signaling from the radio base station for the data flow, or upon receiving an explicit indication from the radio base station that the current buffered data volume at the radio base station for the data flow is below a second defined threshold; wherein the recovery flow control mode includes controlling the first transmit rate via a control algorithm that, for each control cycle, which includes receiving a new instance of the measurement signaling from the radio base station, computes a new value for the first transmit rate in dependence on a difference between the current buffered data volume and a reference data volume that is computed by the radio network controller from a defined target buffer dwell time and a difference between the current value of the first transmit rate and the current value of the second transmit rate.
 2. The method of claim 1, wherein the method provides hysteresis between activating and deactivating the recovery flow control mode by defining the second threshold lower than the first threshold.
 3. The method of claim 1, wherein the data flow comprises one among a plurality of data flows, which may be concurrent or separated in time and which may involve the same user equipment or multiple user equipments, and wherein the method further comprises applying a different instance of the recovery flow control to each data flow for which the radio network control receives corresponding measurement control signaling.
 4. The method of claim 1, wherein the radio network controller maintains an outgoing buffer for buffering data belonging to the data flow, for sending to the radio base station over the transport network, and wherein the method further includes, while recovery flow control is active for the data flow, calculating, in each control cycle, a used data volume representing an estimate of the amount of data that will be transmitted for the data flow from the radio base station to the user equipment during the control cycle, and discarding from the outgoing buffer an amount of data that exceeds the used data volume.
 5. The method of claim 1, wherein activating the recovery flow control mode at the radio network controller for the data flow comprises changing from an Active Queue Management (AQM) mode of operation for the data flow to a recovery flow control mode for the data flow, wherein, while operating in the AQM mode for the data flow, the first transmit rate depends on queue management at the radio network controller.
 6. The method of claim 1, wherein the control algorithm comprises a superposition of a feedforward control component that operates on an open-loop basis and a feedback control component that operates on a closed-loop basis.
 7. The method of claim 1, wherein use of the reference data volume as a control input results in linear time invariant control of the first transmit rate by the control algorithm.
 8. A method of operation in a radio base station, the method comprising: receiving data sent by a radio network controller at a first transmit rate over a transport network coupling the radio base station to the radio network controller, the data belonging to a data flow between the radio network controller and a user equipment; transmitting data for the data flow over a radio interface to the user equipment at a second transmit rate, including accumulating data for the data flow in a buffer at the radio base station in dependence on a difference between the first transmit rate and the second transmit rate; and responsive to a buffered data volume for the buffer exceeding a defined first threshold, activating measurement signaling reporting to the radio network controller, the radio network controller being configured to activate recovery flow control for the data flow in response to receiving the measurement signaling, wherein the radio network controller controls the first transmit rate to drive the buffered data volume towards a reference data volume; wherein the measurement signaling reporting includes sending measurement signaling at successive instances while a buffer dwell time of accumulated data in the buffer exceeds a target buffer dwell time or the buffered data volume exceeds a second defined threshold lower than the first defined threshold, the measurement signaling indicating, for each reporting instance, a current value of the second transmit rate, the current buffer dwell time, and the current buffered data volume; and wherein the method further includes ceasing the measurement signaling reporting responsive to the buffered data volume falling below the second defined threshold or the buffer dwell time falling below the target buffer dwell time.
 9. The method of claim 8, wherein sending the measurement signaling at successive instances comprises sending the measurement signaling periodically, each period defining a reporting period at the radio base station and corresponding to a respective control cycle for the recovery flow control at the radio network controller.
 10. The method of claim 8, wherein sending the measurement signaling at successive instances comprises, for each instance, sending the measurement signaling via defined uplink signaling contained in an uplink frame protocol.
 11. A method of operation in a radio network controller, the method comprising: controlling, in a first mode of operation, a first transmit rate used for transmitting data over a transport network to a radio base station for transmission over a radio interface to a user equipment in dependence on capacity allocation control messages sent from the radio base station, or in dependence on the rate at which the data is incoming to the radio network controller, the data belonging to a data flow between the radio network controller and a user equipment served via the radio base station; controlling, in a second mode of operation, the first transmit rate according to a control algorithm that, in view of measurement signaling sent from the radio base station indicating current values for the second transmit rate, along with a buffered data volume and a buffer dwell time for a buffer used at the radio base station for accumulating data for the flow for transmission to the user equipment, drives the first transmit rate towards a value that minimizes the error between the buffered data volume and a reference data volume; and switching from the first mode of operation to the second mode of operation in response to receiving the measurement signaling.
 12. A radio network controller configured for operation in a wireless communication network and comprising: communication circuitry configured to send data over a transport network to a radio base station for transmission over a radio interface to a user equipment, the data belonging to a data flow targeting the user equipment and accumulating in a buffer at the radio base station in dependence on a difference between a first transmit rate used by the radio network controller for sending data belonging to the data flow to the radio base station over the transport network and a second transmit rate used by the radio base station for transmitting data from the buffer over the radio interface to the user equipment; and processing circuitry configured to: activate and maintain a recovery flow control mode at the radio network controller for the data flow responsive to receiving measurement signaling from the radio base station for the data flow, the radio base station being configured to begin sending and continue sending the measurement signaling responsive to the accumulated data exceeding a first defined threshold, each instance of the measurement signaling indicating a current buffered data volume and a current buffer dwell time for the accumulated data in the buffer, and a current value of the second transmit rate; and deactivate the recovery flow control mode at the radio network controller for the data flow responsive to cessation of the measurement signaling from the radio base station for the data flow, or upon receiving an explicit indication from the radio base station that the current buffered data volume at the radio base station for the data flow is below a second defined threshold; wherein the recovery flow control mode includes the processing circuitry controlling the first transmit rate via a control algorithm that, for each control cycle, which includes receiving a new instance of the measurement signaling from the radio base station, computes a new value for the first transmit rate in dependence on a difference between the current buffered data volume and a reference data volume that is computed by the radio network controller from a defined target buffer dwell time and a difference between the current value of the first transmit rate and the current value of the second transmit rate.
 13. The radio network controller of claim 12, wherein the processing circuitry is configured to provide hysteresis between activating and deactivating the recovery flow control mode by defining the second threshold lower than the first threshold.
 14. The radio network controller of claim 12, wherein the data flow comprises one among a plurality of data flows, which may be concurrent or separated in time and which may involve the same user equipment or multiple user equipments, and wherein the processing circuitry is configured to apply a different instance of the recovery flow control to each data flow for which the radio network control receives corresponding measurement control signaling.
 15. The radio network controller of claim 12, wherein the processing circuitry is configured to maintain an outgoing buffer for buffering data belonging to the data flow, for sending to the radio base station over the transport network, and wherein, while recovery flow control is active for the data flow, the processing circuitry is configured to calculate, in each control cycle, a used data volume representing an estimate of the amount of data that will be transmitted for the data flow from the radio base station to the user equipment during the control cycle, and discard from the outgoing buffer an amount of data that exceeds the used data volume.
 16. The radio network controller of claim 12, wherein the processing circuitry is configured to activate the recovery flow control mode at the radio network controller for the data flow by changing from an Active Queue Management (AQM) mode of operation for the data flow to a recovery flow control mode for the data flow, wherein, while operating in the AQM mode for the data flow, the processing circuitry is configured to control the first transmit rate in dependence on queue management at the radio network controller.
 17. The radio network controller of claim 12, wherein the processing circuitry is configured to implement the control algorithm as a superposition of a feedforward control component that operates on an open-loop basis and a feedback control component that operates on a closed-loop basis.
 18. The radio network controller of claim 12, wherein use by the processing circuitry of the reference data volume as a control input results in linear time invariant control of the first transmit rate by the processing circuitry according to the control algorithm.
 19. A radio base station configured for operation in a wireless communication network, the radio base station comprising: first communication circuitry configured to receive data sent by a radio network controller at a first transmit rate over a transport network coupling the radio base station to the radio network controller, the data belonging to a data flow between the radio network controller and a user equipment; second communication circuitry configured to transmit data for the data flow over a radio interface to the user equipment at a second transmit rate, the radio base station configured to accumulate data for the data flow in a buffer at the radio base station in dependence on a difference between the first transmit rate and the second transmit rate; and processing circuitry configured to, in response to a buffered data volume for the buffer exceeding a defined first threshold, activate measurement signaling reporting to the radio network controller, the radio network controller being configured to activate recovery flow control for the data flow in response to receiving the measurement signaling, wherein the radio network controller controls the first transmit rate to drive the buffered data volume towards a reference data volume; wherein the processing circuitry is further configured to: carry out the measurement signaling reporting by sending measurement signaling at successive instances while a buffer dwell time of accumulated data in the buffer exceeds a target buffer dwell time or the buffered data volume exceeds a second defined threshold lower than the first defined threshold, the measurement signaling indicating, for each reporting instance, a current value of the second transmit rate, the current buffer dwell time, and the current buffered data volume; and cease the measurement signaling reporting responsive to the buffered data volume falling below the second defined threshold or the buffer dwell time falling below the target buffer dwell time.
 20. The radio base station of claim 19, wherein the processing circuitry is configured to send the measurement signaling periodically, each period defining a reporting period at the radio base station and corresponding to a respective control cycle for the recovery flow control at the radio network controller.
 21. The radio base station of claim 19, wherein the processing circuitry is configured to send the measurement signaling at successive instances by, for each instance, sending the measurement signaling via defined uplink signaling contained in an uplink frame protocol. 